1. Field of the Invention
The present invention relates to a protective sheet for a cassette holding a magnetic recording tape.
2. Description of the Prior Arts
Recently, many cassettes holding a magnetic recording tape have been prepared by molding a hausing part of a cassette with a transparent resin or a colored transparent resin in view of a function for a confirmation of a residual portion of the tape and a good appearance. In such transparent cassette, it is usual to place a protective sheet such as transparent plastic sheet between a side surface of a wound tape and an inner surface of a hausing of the cassette so as to form a uniform winding and to rotate smoothly the tape in the cassette. Thus, the transparent protective sheet is easily an abrasion by a friction caused by direct contact with an end of the magnetic recording tape and a core hub of the tape.
The abrasion of the transparent protective sheet cause a deterioration of transparency of the protective sheet whereby the easy confirmation of the residual tape in the cassette having the transparent hausing part and the good appearance as the desired purposed of the transparent hausing part is reduced and moreover a powder of the protective sheet caused by the abrasion is adhered on a running tape to cause drop-out. Moreover, the friction between the magnetic tape and the protective sheet is increased to cause an increase of tape winding torque and an increase of flatter whereby important recording and reproducing characteristics are deteriorated.
A protective sheet prepared by dispersing an abrasive in polytetrafluoroethylene is disclosed in Japanese Patent Publication No. 10484/1976. The protective sheet is not transparent and moreover, the preparation of the protective sheet is not easy and is expensive to remarkably wear the screw for blending. In the protective sheet, hard abrasive grains are incorporated inside of the protective sheet to reduce abrasive grains placed on the surface of the protective sheet with which the magnetic recording tape is directly brought into contact.